if you can mount or otherwise access your target machine's file system, you can simply run first (and second, if you don't want to do any debugging there) stage setup, then replace the files on the target disk with the ones you built above

alternatively, replace the corresponding files in the ISO

Prepare RectOS (Method 2): build the whole MSVC bootcd: nmake bootcd. Note that some things might not work with a MSVC bootcd.

Now start WinDbg, and choose File->Kernel Debug (Ctrl+K)

Set up WinDbg according to how your target outputs its debug messages:

to use a Pipe, check the Pipe option, set Port to \\.\Pipe\TheNameOfYourPipe

to use a com0com serial port, set Port to the name of the port, such as \\.\CNCA0

to use a physical serial port, set Baud Rate to 115200 and Port to the name of the port, such as \\.\COM1

Boot up the target, and select the ReactOS (Debug) boot menu entry. Debug mode is default if you start 1st stage setup.

Enjoy debugging with WinDbg. Some tips:

break in with Tab+K works, just like with kdbg. This is useful since ROS sometimes doesn't react to WinDbg's break-in